This invention relates to monolithic ceramic capacitors having high Q values and near zero temperature coefficients of capacity and more particularly to a low temperature firing magnesium-titanate based ceramic suitable for use with low cost low melting alloy buried electrodes.
Magnesium-titanate has been used as a capacitor dielectric providing a low dielectric constant (K), a low temperature coefficient of capacity (TCC) and a high quality factor (Q). The Q of a capacitor is defined as the ratio of capacitive reactance to equivalent series resistance at the frequency of measurement. The dissipation factor DF is the reciprocal of the Q. It is also known to combine a nonreducible magnesium titanate with a nonreducible alkaline earth borate to form a body of a monolithic ceramic capacitor with co-fired copper electrodes, as is taught by Burn in U.S. Pat. No. 4,308,570 issued Dec. 29, 1981, and assigned to the same assignee as is the present invention. Those capacitors are fired in a partial oxygen atmosphere of 10.sup.-11 O.sub.2 atmospheres at 1050.degree. C. to avoid melting the copper and oxidizing the copper. The nonreducible glass-ceramic is capable of keeping its oxygen at such a firing and therefore does not become semiconducting at all which would typically first show up as a degradation in the Q of the capacitor.
However, even though the Q of such low cost capacitors is quite high, around 3500 at 1 MHz, there are some applications for which much higher capacitor Q's are required.
It is therefore an object of the present invention to provide a monolithic ceramic capacitor having a quality factor Q of at least 5000 at 1 MHz.
It is a further object of this invention to provide such a capacitor having a TCC of less than about 60 ppm/.degree.C. from -55.degree. C. to +125.degree. C.
It is yet a further object of this invention to provide such a capacitor having an air firable construction using a magnesium-zinc titanate and an optimum but low amount of glass for the dielectric body.